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    Mantle upwelling, melt generation, and magma transport beneath mid-ocean ridges

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    Magde_thesis.pdf (30.73Mb)
    Date
    1997-03
    Author
    Magde, Laura S.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/5722
    Location
    East Pacific Rise
    Reykjanes Ridge
    DOI
    10.1575/1912/5722
    Keyword
     Mid-ocean ridges; Plumes; Structural geology; Plate tectonics; Submarine geology; Mantle 
    Abstract
    The formation of new oceanic crust is the result of a complex geodynamic system in which mantle rises beneath spreading centers and undergoes decompression melting. The melt segregates from the matrix and is focused to the rise axis, where it is eventually intruded and/or erupted to form the oceanic crust. This thesis combines surface observations with laboratory studies and geodynamic modeling to study this crustal-production system. Quantitative modeling of the crustal and mantle contributions to the axial gravity and topography observed at the East Pacific Rise shows that the retained melt fraction in the mantle is small (<3%) and is focused into a narrow column extending up to 70 km beneath the ridge axis. Consistent with geochemical constraints, the extraction of melt from the mantle therefore appears to be efficiently focus melt toward the ridge axis. A combination of laboratory and numerical studies are used to constrain the pattern of mantle flow beneath highly-segmented ridges. Even when the buoyant component of mantle flow is constrained to be two-dimensional, laboratory studies show that a segmented ridge will drive three-dimensional mantle upwelling. However, using reasonable mantle parameters in numerical models, it is difficult to induce large-amplitude three-dimensional mantle upwelling at the relatively short wavelengths of individual segments (~50 km). Instead, a simple model of three-dimensional melt migration shows that the observed segment-scale variations in crustal thickness can be explained by focusing of melt as it upwells through a more two-dimensional mantle flow field. At the Reykjanes Ridge, the melt appears to accumulate in small crustal magma chambers, before erupting in small batches to form numerous overlapping hummocky lava flows and small volcanoes. This suggests that crustal accretion, particularly at slow-spreading centers, may be a highly discontinuous process. Long-wavelength variations in crustal accretion may be dominated by variations in mantle upwelling while short-wavelength, segment-scale variations are more likely controlled by a complex three-dimensional processes of melt extraction and magma eruption.
    Description
    Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1997
    Collections
    • Geology and Geophysics (G&G)
    • WHOI Theses
    Suggested Citation
    Thesis: Magde, Laura S., "Mantle upwelling, melt generation, and magma transport beneath mid-ocean ridges", 1997-03, DOI:10.1575/1912/5722, https://hdl.handle.net/1912/5722
     

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